As the printing methods suitable for non-absorbent substrates, gravure printing for soft packaging materials, flexographic printing for sanitary materials, silk screen printing for metal plates, inkjet printing for indoor and outdoor advertisement, and the like, are generally known. Ink compositions used for these printing methods generally make use of solvents being toxic and teratogenic, such as toluene, ethyl acetate, methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone. These solvents have been difficult to handle, such as in terms of the specification of ambient concentration, installation of draft chamber due to unpleasant odor or the like, or occurrence of an obligation to take periodic health examinations. However, there has been a demand for an ink having higher safety and sanitariness. In particular, unlike the gravure printing, flexographic printing and silk screen printing, which are performed in dedicated factories, inkjet is used in general workplaces such as business offices, and therefore, more consideration should be taken with regard to safety, noxiousness and odor.
For an enhancement of safety, in regard to the gravure printing for flexible packaging materials and flexographic printing for sanitary materials, non-toluene inks which do not use toluene having a low set value for ambient concentration, or water-based inks which contain alcohol as a main component, have been developed.
Meanwhile, also in regard to the inkjet printing, an ink having no toxicity and teratogenicity, which utilizes a polyalkylene glycol-based solvent or a hydrocarbon-based solvent, has been developed (Patent Document 1). However, since such inks formed from polyalkylene glycol-based solvents or hydrocarbon-based solvents are not capable of dissolving the surface of printing substrates, there have been problems of having poor adhesion and weather resistance.
Thus, recently, a solvent-based inkjet ink having improved adhesion, weather resistance and the like, has been developed by using a nitrogen-containing heterocyclic compound such as 2-pyrrolidone or N-methyl-2-pyrrolidone, a sulfur-containing compound such as DMSO, or a lactone-based compound, which does not have toxicity and teratogenecity as is the case with polyalkylene glycol-based solvents, as an assistant solvent (Patent Document 1, Patent Document 2). However, since such an ink has strong dissolvability, there have been problems such as dissolution of pigments, corrosion of print head materials, and strong bad odor.
That is, there has been hitherto a demand for a non-aqueous inkjet ink which smells less, has excellent safety and sanitariness, excellent resin dissolvability, excellent printing stability and excellent dryability, causes no corrosion of printers, and has good adhesiveness to non-absorbent substrates.    Patent Document 1: Japanese Patent Application (Laid-Open) No. 2005-330298    Patent Document 2: Japanese Patent Application (Laid-Open) No. 2000-515920.